With little of the usual fanfare, London mayor Boris Johnson recently published his London Plan, a 300-page document that will determine the capital's growth development for the rest of this decade, and beyond.

The plan links the mayor's long-term policies in the key areas of housing, transport, economic development and the environment. But where the plan, which I developed with former Labour mayor Ken Livingstone, took London forward, Johnson's version takes us backwards.

Nothing in the mayor's plan has generated as much controversy as his housing policies. Housing is central to the quality of Londoners' lives and it is inextricably linked to the success of our policies on education, health and social mobility.

The first issue is the supply of new homes: Johnson's overall housing target is too low. The mayor said that his London Plan would ensure "there are enough homes" for the city. But the small print reveals that his target of 32,210 new homes every year is almost 2,700 below the figure the plan itself claims is needed. The planning inspector who assessed Johnson's plans found that London actually needs between 34,900 to 37,400 homes a year – way above the mayor's target.

The greatest need in London is for new, social rented homes. With rising rents and homelessness, 360,000 households on London's waiting lists and 750,000 people living in overcrowded conditions, that need is acute. This is where the mayor's ideology trumps what the evidence says is needed.

Since deciding to run for City Hall, Johnson has made a point of attacking Livingstone's policy that 50% of all new residential developments across London should be affordable and that 70% of those should be for social rent. The importance of this 50% target, first introduced in 2004, cannot be underestimated.

One of Johnson's first moves was to scrap the target, a move that has been roundly criticised by tenants groups, local councils, developers and – crucially – the independent inspectors who examined the London Plan. Local authorities and developers can now decide how many affordable homes they provide and how many (or few) of them should be for social rent. In a nutshell, the scrapping of the London-wide target removes the only mechanism compelling developers to deliver affordable housing on individual sites across London.

The inspectors' report pressed home "the importance attaching to consistency and certainty when negotiating for site acquisition and progressing schemes with borough development managers over relatively long time periods".

Johnson claimed that Livingstone's 50% target "comprehensively failed London", but the evidence suggests otherwise. When introduced, the policy had an immediate impact, and by 2008 affordable housing completions had risen to 36.9% of all housing, at a time when house building itself was at an all time high.

As pointed out at the examination of the London Plan, the "50% target is only now beginning to be reflected in development plan documents and in actual delivery". The mayor's approach is not one underpinned by evidence. We see this again in his approach to mixed and balanced communities – one of the distinctive hallmarks of our capital. They are absolutely central to a more equal and just society.

Johnson himself has warned about the effects of the government's welfare proposals, saying that the "great genius" of Londonis that it is a place "where people of many different economic groups live together. The last thing we want to have in our city is a situation like Paris where the less well off are pushed out to the suburbs."

Quite so, but the reality is that his own housing policies will achieve the same results as the benefit caps he warned against and the government's new affordable rent model. The mayor wants private, market housing to be built in areas with lots of social housing. But his plan does not seek new social housing in areas with lots of private homes.

The mayor dismisses the independent inspectors' warning that he will not achieve universally balanced and mixed communities this way and that the discriminatory element of the policy should be dropped. The inspectors said "we do not share the mayor's view that there is no need to stimulate social rented housing" in areas of mono-tenure market housing.

The results of this approach are already emerging. Johnson is giving the green light to boroughs to remove social rented homes in regeneration schemes. On his watch, the four worst performing inner-London boroughs for affordable housing are those where mono-tenure private market housing is most prevalent – Hammersmith & Fulham, Kensington and Chelsea, Wandsworth, and Westminster. So poor is their record that Wandsworth and Hammersmith & Fulham have only started 46 and 53 social rented homes respectively between 2008 and 2010.

These London Plan policies have not changed since Johnson first published his consultation draft in October 2009. They set the path for the coming government's housing and welfare policies. Together they are all leading in one direction – a wholesale attack on social housing and social segregation on an unprecedented scale.

Nicky Gavron is a member of the London Assembly and Labour Group spokesperson for housing and planning

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